In recent years, there has been increasing discussion concerning the effect of greenhouse gases on the Earth's atmosphere. An increase in greenhouse gases can lead to climate changes, also known as the “greenhouse effect,” and these changes are primarily caused by the increase of the carbon dioxide concentration in the atmosphere. The burning of fossil fuels, such as natural gas, mineral oil and coal, is the primary cause of this increase in carbon dioxide concentration. Most greenhouse gases, including carbon dioxide, take a long time to leave the atmosphere. Therefore, there is an increased interest in developing processes that can remove carbon dioxide and other greenhouse gases from the atmosphere on an industrial scale. For example, carbon dioxide can be removed from the flue gases of fossil fuel-fired power plants or from the exhaust gases of gas turbine power plants before they are released into the atmosphere.
Carbon dioxide is generally separated using a CO2 capture system such as an absorption bed. Other techniques currently being investigated include pressure swing adsorption, temperature swing adsorption, gas separation membranes and cryogenics. Absorption is a process that occurs when a gas or liquid solute is taken up by the volume of a solid, liquid, or gas called an absorbent. Pressure swing absorption relies on the principle that under pressure, gases tend to be transferred to the volume, or absorbed. The higher the pressure, the more gas is absorbed. When the pressure is reduced, the gas is released, or desorbed. Temperature swing absorption relies on the principle that at low temperatures gases tend to be absorbed and at higher temperatures, gases tend to be desorbed. Pressure-temperature swing absorption combines both temperature and pressure variations to maximize absorption of gases into the absorbent and to minimize the time required for desorption of the gases from the absorbent.
The absorption process is generally run in multi-bed systems so that when some absorption beds are operating in the absorption step, the other beds are being regenerated in the desorption step. Current absorption technology used for separating CO2 from gas mixtures typically consumes about 10 to about 100 times the theoretical minimum energy required for the separation. This energy is consumed during pressurization or heating of the gas stream or absorbent material.
CAES systems are used to efficiently capture electrical energy during hours of off-peak energy consumption using pressure and temperature swings. In a CAES system, one or more electrically activated compressors uses electricity during off-peak energy consumption periods to compress gas that is then stored as potential energy. The process of compressing the gas releases heat and can cause the gas stream to reach temperatures ranging from about 300° C. to about 500° C. The gas stream is usually cooled after the compression process using a heat transfer system. To increase the efficiency of the CAES system, it is known that a heat accumulator may be used to capture heat from the compressed gas stream and to store it in a medium, such as stone, so that it can be used to reheat the compressed gas stream during expansion.
The compressed gas is then stored in a CAES reservoir, usually an underground cavern or other underground geologic formation, until it is used to produce electricity during periods of peak energy consumption. The compressed gas is converted back to electricity by expanding it through a turbine. The process of expansion significantly cools the gas stream, which can damage or freeze the expansion turbine. To counteract the temperature drop of the gas stream during expansion, the compressed gas stream is often preheated through the combustion of fuel. It is also known that the compressed gas stream can be preheated by passing it, in the opposite direction, through the heat transfer system that is used to cool the gas stream during compression.
Currently, there are two CAES systems that have been constructed and are in operation. The first, built in 1977, is in Germany and has a storage capacity of 300,000 m3 and can generate 290 MW of electricity for two hours. The second, built in 1991, is in Alabama and has a storage capacity of 540,000 m3 and can generate 110 MW of electricity over a period of 26 hours. Another CAES plant is being planned in Ohio that will use an abandoned limestone cavern with a storage capacity of approximately 10 million cubic meters with an operating pressure of about 50-100 atm.
U.S. Patent Application Publication No. 20060260312 A1 to Ingersoll discloses a method of creating liquid gas using a wind energy system that has a plurality of direct compression wind turbine stations. In Ingersoll, wind energy is collected and stored as compressed air and liquid gas. The purpose of the Ingersoll disclosure is to more reliably and cost effectively deliver power from existing and proposed offshore wind energy plants.
A need exists, for example, to provide improved methods of removing CO2 and other greenhouse gases from power plant flue gases and other mixed gas streams. There is a further need for improving the efficiency of CO2 removal by combining absorption with existing industrial processes having similar gas handling and temperature and pressure cycling behaviors.